Line testing is typically employed in communication networks where terminal devices located in the premises of a customer and therefore also referred to as Customer Premise Equipment (CPE) is connected by means of a wired communication line with a central termination of the subscriber line which is typically a Central Office (CO). An example is the Public Switched Telephone Network (PSTN) where the terminal devices like telephones and faxes are connected to the Central Office by means of copper twisted pairs, which are typically called the “Tip” and “Ring” wires. The “Tip” and “Ring” wires are sometimes called also the A-B wires.
Other data communication services may be provided over the copper wires such as Integrated Digital Networks (ISDN), Digital Subscriber Lines (DSL) such as ADSL (Asymmetric DSL), VDSL (Very High DSL) etc., or All Digital Loops (ADL). Central Offices of communication networks are usually equipped with a device for performing a metallic line testing. The metallic line testing may be a sequence of electrical measurements which are repeated for each subscriber line with a particular frequency for example every 12 hours. The goals of the line testing is to determine information on the condition of the line such as to detect if the line is broken or affected by leakage or corrosions, to detect if a Foreign Electromotive (FEM) Voltage or any external disturbance is coupled with one or both of the Tip-Ring wires of a copper pair, to measure a loop length, to measure the typical cable parameters like the capacity between the two wires “Tip” and “Ring” of the communication line or the capacity between the Tip wire and Ground and between the Ring wire and Ground. Other goals include the detection of a DSL signature or passive probe termination circuits on the CPE side, to measure the ringer load which basically includes to detect how many telephones are connected in parallel to the same subscriber loop, to monitor the loop current in different states of the loop such as when the telephone is in Off-Hook, On Hook etc., or to perform tests in parallel with an ongoing DSL transmission without impacting its performances in terms of data rate and transmission errors.
In some situations of a failure, the Subscriber may call the Hotline of the Service Provider or the local Telecom and complain about the communication problems. The operator may then run on-the-fly a Line Testing on that particular subscriber line loop by means of dedicated software which is typically referred to as the “Expert System”. The results of the measurements will be displayed after some time at the Operators computer so that he can determine the situation and condition of the line including where the problem takes place, if it is necessary to provide a technician for the on-site maintenance or repair etc.